The present invention relates to a nonconstrained prosthetic implant for the replacement of metatarsophalangeal joints.
The replacement of degenerated natural joints with man-made prosthetic replacements is well known, including the replacement of the metatarsophalangeal joint (MPJ) of the great toe. One such replacement, to which the present invention relates, uses a two-piece, nonconstrained prosthesis to replace the first MPJ where the metatarsal head is resected and replaced with a metal component and the proximal phalanx is resected and replaced with a metal-backed polyethylene component.
The early implants for the great toe were hemiarthroplasty devices, where only one side of the joint was replaced-either the metatarsal head or the proximal phalanx. As early as the 1950's, one Swanson developed a metal replacement for the metatarsal head, fixed with an intramedullary stem. The procedure was deemed unsuccessful, and failure was attributed to resorption of bone around the stem. In the mid-1960's, Swanson became aware of a medical-grade silicone developed by Dow Corning. First, he tried replacing the metatarsal head with the material, but because of high shear and compressive loads across the first MPJ during gait, the prosthesis failed. Because of the failures incurred by using a hemiarthroplasty on the metatarsal side, he turned his attention to the proximal phalanx. He designed a silicone implant having an intramedullary stem and proximal portion that acted as a spacer, replacing the bone which is resected in a Keller procedure. Because the implant was only acting as a spacer, it lacked full functional motion and caused severe reactions of the surrounding bone due to debris thought to be generated by wear between the implant and metatarsal head.
In the 1970's, a double-stem hinged implant similar to one for the metacarpophalangeal joint (base of the finger) was developed using a new High Performance Silastic Elastomer. Various forms of such constrained, silastic implants have been in use ever since, and they have been relatively successful. Pain relief is obtained in almost all cases, patients have the ability to walk, and functional scores have been shown to increase. While the range of motion has increased compared to the other available options (fusion, Keller procedure), a fully functional toe has not been the result. Often, due to the constraining nature of the devices, the implant pistons in and out of the medullary canal, causing debris formation and synovitis. In view of the recent silicone breast implant scare, there is now much concern about the use of silicone-based implants in the hand and foot. Thus, the two-piece metal on poly designs that have worked so well in the hip and knee areas are now being considered by many for MPJ replacements.
The somewhat unconstrained implants on the market today have a metatarsal component (generally cobalt chromium alloy) with a convex articulating surface that mates with a concave medical-grade ultra high molecular weight polyethylene (UHMWPE) metal-backed (generally titanium alloy) phalangeal component. Both components utilize an intramedullary stem for stability and fixation purposes.
A great toe replacement, developed by one Richard Koenig, DPM, and marketed under the trademark Biomet, utilizes a medial surgical approach to address the plantar articulation of the sesamoids with the plantar surface of the metatarsal head. The metatarsal component wraps around the dorsal to plantar aspect of the metatarsal head. This replacement, with its somewhat difficult surgical implantation technique, was introduced in the late 1980's. The dorsal flange of this replacement allows a normal range of dorsiflexion, but may cause interfered motion of the sesamoid/plantar metatarsal articulation. While there is unconstrained motion from dorsal to plantar, the motion of this implant is in fact semi-constrained because of the flatter radius of curvature medial to lateral, which inhibits the motion.
In the early 1990's, podiatrist Kerry Zang developed a simpler replacement which is available from MicroAire Surgical Instruments, Inc. of Valencia, Calif., under the trademark Bio-Action. Neutral cuts (generally perpendicular to the long axis of the bone) are made on both the phalanx and metatarsal head. The metatarsal head is replaced with less than a hemisphere of a somewhat spherical surface, while the phalangeal base is covered by a circular metal-backed polyethylene insert. The range of motion during walking is limited due to the absence of a dorsal flange; however, the sesamoid/metatarsal articulation is relatively undisturbed. This implant, due to its generally spherical surface, is unconstrained. Its medial to lateral alignment is maintained by the soft tissue and not by the implant design.
In early 1993, another great toe implant became available from Acumed. This implant combines some of the positive aspects of the two previously discussed implants--the dorsal flange of the Biomet-Koenig implant with the noninvasive plantar aspect of the Bio-Action implant. The Acumed toe is similar to the Koenig implant because it is also semi-constrained due to a flatter radius limiting the motion in the medial to lateral direction. Both the Acumed and Biomet toes require a proper, perpendicular alignment of the neutral cut with the convex surface of the metatarsal implant component.